In tribute to his grandfather, who served in the British army, director Peter Jackson ("Lord of the Rings") gives us “They Shall Not Grow Old,” a moving documentary portrait of the soldiers who fought in World War I.

Built from a generous amount of film footage from the era, “They Shall Not Grow Old” traces a chronological journey through the war from the perspective of the soldiers, from their recruitment, through their training and finally to the battlefield in France.

Warner Bros. Pictures

A restored and colorized image shows a moment from Peter Jackson’s World War I documentary “They Shall Not Grow Old.”

Rather than go with a consistent narrator, the lengthy cycle of clips is attended by numerous voiceovers — different men reading the words of the soldiers themselves (many of whom were under age and had to lie their way into service). The effect, combined with the primitive, intimate nature of the footage, almost makes “They Shall Not Grow Old” feel like an extended home movie, narrated in first person by its characters.

Early on we see new recruits, still in civilian clothing, marching in formation, juxtaposed against fliers and posters lobbying volunteers to the war effort. Period footage gives us a taste of what life was like circa 1914, as the war broke out (including an amusing anecdote about how a group of British and German rugby players, dining together as part of an event, reacted to news of the war).

Once we arrive on the battlefield, Jackson kicks in with a unique visual style, colorizing and enhancing the archival footage in a way that attempts to put the audience more fully in the moments they are watching onscreen. Focused primarily on the action at the war’s Western front in rural France, “They Shall Not Grow Old” paints an increasingly bleak portrait of a shelled and increasingly lifeless battlefield.

Warner Bros. Pictures

The progression from the original to the final colorized image in a moment from Peter Jackson’s World War I documentary “They Shall Not Grow Old.”

For a time, we learn about the different aspects of stagnant trench warfare. We see the design and layout of the trenches themselves, and get a sense of the perpetual cycle of shelling, gas attacks and the treacherous threat of mines soldiers lived with. We also get a rather unflinching look at a reality where soldiers sit on communal logs to defecate, and constantly try to fight off the rats that come to eat the dead bodies that litter the battlefield.

“They Shall Not Grow Old” never quite matches the graphic carnage of a “Saving Private Ryan,” and the poor quality of the archival footage — even enhanced — saves audiences from the full impact of their visuals. But Jackson’s film is still very vivid, heartbreaking and disturbing, with numerous images of dead bodies and the aftermath of battles. The film doesn’t make for a hard R-rating, but it isn’t for the squeamish, either.

The narrative eventually builds to a peak as the soldiers prepare to go “over the top” of the trenches into a massive offensive in the “no man’s land” between the military factions, and the film’s third act offers a powerful finale. Overall, “They Shall Not Grow Old” is a moving and valuable tribute to a war that, comparatively speaking, doesn’t get so much attention on the big screen. Jackson’s effort is welcome, even if it’s hard to watch.

Rating explained:“They Shall Not Grow Old” draws an R-rating for various images of the aftermath of battle — mainly dead bodies of soldiers and animals. There are also some stills of bare backsides and some mild profanity.